This invention relates to an improved air knife such as is used, for example, in coating a traveling paper web. It is very important that the gap between the lips defining the air knife orifice be accurately maintained at a uniform distance across the entire width of the orifice. Since the gap is normally quite narrow (i.e. about 0.030 inch), such factors as small deflections of the lips, layers of built-up coating from the coating operation and material removed from the lips when cleaning them, can cause relatively great changes in the orifice gap. This is detrimental to the function of the air knife which is to direct a steady, uniform jet of air against a freshly coated web to evenly meter and smooth the coating on the web across its entire width.
A problem typical of prior air knives is that they were constructed with adjustable screws extending within the body member between their lips. This necessitated the air to flow around the adjusting screws before leaving the slotted orifice. Such an arrangement is shown in U.S. Pat. No. 2,139,628. The adjustment screws caused some interruption or turbulence in the stream of air leaving the orifice across the air knife width.
Another problem ancillary to the prior design, which in essence is still being used, relates to the manner in which the gap between lips is adjusted to remain constant in the cross machine direction. Over a period of time, coating builds up on the lips and is removed by scraping. This produces slight deformities on the surfaces of the lip tips which is compensated for by tightening the adjustment screws extending between the lips. Eventually, the lips become warped from the constant forces exerted by the adjustment screws. In addition, after repeated adjustments, the threaded contact between the screws and their mounting in the lips becomes loose which makes it more difficult than ever to maintain the lip gap within predetermined limits. At the final stages of such looseness in the threaded mountings, the lips will move rapidly enough to chatter at which time all semblance of accurate control of the lip gap vanishes.
When the lips of such prior design air knives had been worn by several scrappings and deformed by continued tightening of the adjustment screws on the top lip, they had to be disgarded because they could not be refinished and still maintain the orifice gap within the desired dimensions across the width of the air knife. This is both costly and time consuming since the lips, then as now, are made of stainless steel and must be accurately machined and finished.
Still another problem with such prior air knife designs relates to the fact that the top lip had to be slotted in order to permit both lateral and pivotal adjustment to adjust the gap relative to the stationary lower lip. Even though washers were positioned over the slots, some air would inevitably leak past to mix with the coating material and cause misting.